Few mammalian genes undergo allele-specific methylation - imprinting - that limits their expression to only one parental allele. Imprinting abnormalities lead to only a handful of human diseases, including pseudohypoparathyroidism type Ib (PHP-Ib), which is caused either by maternally inherited microdeletions within or up-stream of GNAS (autosomal dominant PHP-Ib; AD-PHP-Ib), by paternal uniparental isodisomy involving the GNAS locus on chromosome 20q (patUPD20q), or by an as-of-yet undefined genetic defect unrelated to GNAS. Common to these different PHP-Ib variants is a loss of all or some maternal GNAS methylation imprints, which impairs Gs expression from the maternal allele, leading in proximal renal tubules (and few other tissues) to deficiency of Gs, the signaling protein down-stream of the PTH-receptor (PTHR1). This results in hypocalcemia, hyperphosphatemia, and insufficient 1,25(OH)2 vitamin D (1,25D) production despite elevated PTH levels, but no increase in the levels of fibroblast growth factor 23 (FGF23), a recently discovered phosphaturic hormone. Many aspects of the defective regulation of calcium-phosphate homeostasis in PHP-Ib remain undefined, particularly the role of FGF23, which is produced by osteocytes, possibly in response to PTH. However, it remains uncertain whether Gs-signaling is imprinted in these cells or why FGF23 does not prevent the development of hyperphosphatemia. Gs imprinting may also occur in other tissues, including thyroid, pituitary, bone, and brown adipocytes, but it is difficult to explore these PHP-Ib aspects in patients. We therefore generated mice carrying a GNAS deletion (?Nesp55) identified in some AD-PHP-Ib patients. Mice with paternal deletion (?Nesp55p) are healthy and fertile, while maternal inheritance (?Nesp55m) leads to the same GNAS methylation changes as in AD-PHP-Ib and consequently to PTH-resistant abnormalities in mineral ion homeostasis. Surprisingly, the ?Nesp55m animals die by day P5, strongly indicating that Gs expression from the maternal allele alone occurs not only in the proximal renal tubules, but also in other tissues. By crossing ?Nesp55p females with ?XLm males (both are healthy) to generate ?XLp/?Nesp55m offspring, we developed a viable model of AD-PHP-Ib. This model will help define the intricate interplay between PTH, FGF23, 1,25D, and possibly other hormones (Aim 1). We will furthermore determine whether imprinted Gs expression occurs in additional tissues, such as pituitary, thyroid, and possibly some bone cells, and how Gs expression from the non-methylated GNAS allele is silenced through cis- and/or transacting mechanisms (Aim 2). The results of the proposed studies will likely to improve diagnosis and treatment of PHP patients, and will have important implications for the treatment of other imprinting disorders and for common diseases such as osteoporosis and chronic kidney disease.